Study suggests diet of Great Lakes salmon has changed

Charlevoix, Mich. — A study recently published in a scholarly journal highlights why officials reduced salmon stocking in Lake Michigan, as well as how the lake is mimicking trends seen in Lake Huron before the salmon crash occurred there.

Several U.S. Fish and Wildlife Service, U.S. Geological Survey, and Michigan DNR researchers authored a study of Lake Michigan salmon stomachs published by Transactions of the American Fisheries Society in January, comparing what the fish ate in 2009 and 2010 to their diet in 1994-96.

What they found was that salmon shifted from a more diverse diet of larger alewives, bloaters, and rainbow smelt to almost entirely alewives. The alewives they’re eating now are also smaller in general than in the past, according to USFWS researcher Greg Jacobs, who started the study as a grad student with the University of Michigan.

“The biggest thing is the larger alewives … just aren’t out there anymore,” Jacobs told Michigan Outdoor News. “Chinook like big alewives, but they’ve run out of alternatives so they are eating more and more alewives as time passes.”

Researchers collected more than 1,000 salmon stomachs from all states surrounding Lake Michigan in 2009 and 2010, and compared what they found to salmon diet analysis conducted by the DNR in the mid-1990s. Results showed diversity of the fish diets for small salmon (under about 20 inches) shifting from 58 percent alewives in the 1990s to 85 percent alewives in 2009-10. The diet of larger salmon went from 84 percent to 99 percent alewives.

Chinooks in Lake Michigan are not only eating a higher percentage of alewives, but those alewives are smaller on average and contain less nutrition than before, researchers said.

“Smelt are at extremely low levels and bloaters are at extremely low levels so there really isn’t any alternative prey for the salmon,” said Randy Claramunt, a DNR research biologist who helped conduct the study. “It’s bad news and it’s kind of discouraging to see the lack of diversity of prey (species) and big prey.”

Salmon stomach analysis also confirmed what researchers have found in bottom trawls in recent years – that there is a “truncation” or reduced range in the age and size of alewives in Lake Michigan.

Chuck Madenjian, a biologist with the U.S. Geological Survey who contributed to the salmon study, said both the less diverse salmon diet and the truncation of the size of alewives salmon are eating in Lake Michigan mimic trends witnessed in Lake Huron before the salmon population crashed around 2003.

“In Lake Huron they would routinely get 7-, 8-, or even 9-year-old alewives” in bottom trawls, Madenjian said. “That was pretty consistent until about the year 2000 … and then instead the highest age that were sampled were 5-year-olds.

“The highest age we caught in 2000 (in Lake Huron) was 5, and that pattern persisted into 2001 and 2002.”

The same thing is happening in Lake Michigan, only at a slower pace. It’s one of several indicators resource managers considered when they reduced salmon stocking in Lake Michigan to better balance prey fish and salmon populations, he said.

“In 2009 through 2011, the oldest alewives we’ve got were age 6. We used to get 7- and 8-year-olds,” Madenjian said. “And it looks like things aren’t improving, because in 2012 the oldest age we got in the bottom trawl was age 4.